KIP-267 — Add Processor Unit Test Support to Kafka Streams Test Utils

Accepted Kafka 2.0 StreamsTesting

KIP-267 adds a `MockProcessorContext` to the `kafka-streams-test-utils` module so developers can unit-test `Processor`, `Transformer`, and `ValueTransformer` implementations in isolation without starting a full `TopologyTestDriver`. Previously writing such tests required custom mock code for `ProcessorContext` that reproduced complicated internal behavior including state store registration, forwarding capture, and punctuator scheduling.

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